1. Field of the Invention
The invention relates to a method of making pig iron by an in-bath direct reduction, in which zinc is recovered from a dust containing zinc, particularly dust containing zinc from the steel industry such as blast furnace dust or converter dust.
2. Description of the Prior Art
Large quantities of dust containing zinc and iron are produced in the steel industry. It is desirable to recover zinc from it and also to make use of the iron content. Some proposals have been made.
It is known from EP-A-94114, after having separated the coarsest dust particles from blast furnace gas in a so-called dust-catcher, to subject the dust to wet gas scrubbing, after which the slurry is separated by hydro-cycloning into a coarse, low zinc fraction which may be reused in the blast furnace as raw material containing iron, and into a fine, high zinc fraction. The fine fraction has too little zinc to be of interest hitherto for zinc recovery and, depending on prevailing environmental regulations, must be disposed of as waste or stored as chemical waste.
DE-A-3536635 describes a process of recovery of iron, zinc and other metals from steel industry dusts by feeding the dust with oxygen into a so-called melting cyclone in which under reducing conditions an iron oxide containing slag which is free of zinc is formed. The zinc vaporizes. The gas discharged from the melting cyclone is mixed with air and fuel for after-burning, and zinc oxide particles are obtained from the resulting gas by cooling. The iron oxide slag is collected in a chamber under the melting cyclone and fed from these into a steel converter partially filled with pig iron, in which it is reduced by the carbon in the pig iron.
In the context of the present invention discussed below, it should be noted that it is known from NL 257692 to pre-reduce iron ore concentrate in a melting cyclone. The pre-reduced and molten iron ore trickles down the wall of the melting cyclone into a second metallurgical vessel in which, with oxygen and fuel supplied, the pre-reduced iron ore is further reduced into pig iron. During reduction in the second metallurgical vessel, a hot reducing gas containing CO is produced which passes from the second metallurgical vessel into the melting cyclone. With oxygen supplied, the reducing gas is kept combusting in the melting cyclone and this has the effect of pre-reducing and melting the iron ore. Finally the gas is discharged from the melting cyclone as waste gas. Steel Times International (GB), 17(1993) March, No. 2, page 24, also describes this in-bath direct reduction process using a melting cyclone.